Too Little CO2 To End Life On Earth
It is no surprise to anyone who has studied the history of our planet and the life it harbors that CO2 levels have been falling for billions of years. Despite all the hoopla over rising CO2 levels, eventually Earth will have lost so much carbon dioxide from its atmosphere that plants and trees will suffocate, signaling an end to life as we know it. Now, a team of scientists from the California Institute of Technology, led by physicist King-Fai Li, have proposed a way to avert disaster—get rid of much of the atmosphere.
In a paper titled “Atmospheric pressure as a natural climate regulator for a terrestrial planet with a biosphere,” published in the June 1 issue of the Proceedings of the National Academy of Sciences (PNAS), Li et al., suggest that the life span of the biosphere can be extended at least 2.3 billion years, more than doubling previous estimates. Here is a description of the problem from the paper's online abstract:
Lovelock and Whitfield suggested in 1982 that, as the luminosity of the Sun increases over its life cycle, biologically enhanced silicate weathering is able to reduce the concentration of atmospheric carbon dioxide (CO2) so that the Earth's surface temperature is maintained within an inhabitable range. As this process continues, however, between 100 and 900 million years (Ma) from now the CO2 concentration will reach levels too low for C3 and C4 photosynthesis, signaling the end of the solar-powered biosphere.
There are three basic categories of plants when it comes to photosynthesis: C3, C4 and CAM. The difference between them are the ways in which CO2 is extracted from the air and the primary products of photosynthesis. In C3 plants the enzyme responsible for photosynthesis, RUBISCO, is also the enzyme involved in the uptake of CO2. Examples of C3 plans include wheat, barley, potatoes and sugar beets—most plants are of this type.
C3, C4, and CAM type plants.
C4 plants use PEP Carboxylase for uptake, which then supplies CO2 to RUBISCO for photosynthesis. CAM stands for Crassulacean Acid Metabolism. In CAM plants CO2 is stored in the form of an acid before use in photosynthesis. C4 and CAM photosynthesis are both adaptations to arid conditions and result in better water use efficiency. C4 plants include fourwing saltbush (shown in the middle picture above), corn, and many summer annual plants. CAM varieties include cactus, some orchids, bromeliads and agaves. With the end of the “solar-powered biosphere” there may be little food, but at least our distant descendants will be able to make Tequila.
It seems that solar power is not the final answer to all of our earthly problems after all, in fact it is set to become our worst nightmare. Sometime between 100 million and 1 billion years from now, Earth will become a lifeless wasteland. The cause of the biosphere's ultimate demise, however, is not certain—it is a race between CO2 depletion and the rising output from the sun. If plant-life doesn't die out soon enough, the sun is on schedule to fry the planet in about a billion years. As we mentioned in chapter 10 of The Resilient Earth, Varying Solar Radiation, all stars eventually die. Scientists think they know what the ultimate fate of our star will be.
The fate of a star when it leaves the main-sequence, after exhausting its supply of hydrogen, depends partly on its mass. Once a star's hydrogen is spent, it will start burning helium in a nuclear reaction that forms carbon and oxygen. A side effect of this change in nuclear fuel is swelling and cooling of a star's outer layers. When this happens, the star becomes a red giant.
Once helium is exhausted, a large star will burn carbon and oxygen, proceeding with successively heaver elements until iron and nickle are produced. At that point the star runs out of fuel entirely. Stars massing less than eight solar masses are not big enough to burn carbon so their careers as red giants ends when their helium supply is gone. With no ongoing thermonuclear reaction to provide heat to support it, a star will collapse.
When a star the size of the Sun runs out of fuel and collapses, the sudden contraction of matter causes one or more explosions. These explosions blow off the outer layers of the star, forming a planetary nebula. Many of the more spectacular photographs from the Hubble Space Telescope are of planetary nebulae. Over time, the matter in the nebula will disperse, leaving the burned-out remains of the star behind—a white dwarf.
But the death of the sun is not really an immediate concern, humanity will have either perished or have found a new home outside the solar system long before the sun swallows the Earth. As we said in the first chapter of The Resilient Earth:
Earth is doomed, and science tells us how our world will end. In a billion years, the Sun's output will have increased by ten percent above today's levels, causing runaway greenhouse heating and the end of life on Earth. In 5 billion years, the Sun will start to run out of hydrogen and begin to swell. In 6 billion years, the Sun will become a red giant, engulfing Mercury, Venus and Earth. Finally, in 7 billion years, the Sun will eject its outer layers and slide into retirement as a white dwarf. The Sun will spend its final days quietly cooling—a dimming ember in space. Earth, and its vibrant ecology, will have long vanished. This happens to stars and their planets all the time, it is the fate decreed by the laws of nature.
Ignoring the high probability that all life will all be incinerated by the sun in a billion years, what can be done to save Earth from CO2 depletion? As mentioned at the beginning of this article, the Cal Tech team suggests we reduce the thickness of the atmosphere, lowering air pressure to about one-sixth today's pressure at sea level. To do this we would need to develop a technology that sucks nitrogen from the air, since 78% of Earth's atmosphere consists of nitrogen. The overall affect of this thinner air would be oxygen enrichment. Of course, our distant descendants would have to alter their metabolisms, developing a physiology similar to the Sherpa people of Nepal. I doubt that the atmospheric reduction program would achieve its goals quickly, giving Earth's remaining animal life, people included, time to adapt to the new low pressure lifestyle.
Ecologist Kenneth Caldeira of the Carnegie Institution at Stanford, who has studied the implications of low carbon concentrations in the atmosphere, says the researchers have made “a persuasive case” that pressure can play an important role in the planet's long-term atmospheric composition. But he doubts that anyone knows what will happen to atmospheric pressure in the distant future. To that we can add, no one really knows what is going to happen in the near future either.
Is this the Earth of the future? Image by Jeff T Alu.
It is interesting that science has found yet another way for all life on this planet to perish that doesn't require the existence or misbehavior of mankind as its cause. According to the PNAS article, we are threatened by the loss of carbon dioxide, but in real life we are being told to get rid of it. The eventual death of the planetary biosphere due to lack of carbon dioxide tells us some things about the current CO2 induced global warming hysteria. Though carbon dioxide levels have varied widely over the 4.5 billion years of Earth's existence the trend is irrefutably downward. When taken as part of the grand sweep of Earth's history the current rise of atmospheric CO2 levels is not even noteworthy. The forces that have changed Earth's climate and atmospheric composition in the past continue to change it today and will do so until our planet dies. These natural forces are huge, titanic, beyond human comprehension and control.
In the face of such forces, mankind's power to significantly or permanently alter the environment is negligible. Those in government and regulatory agencies should think twice when they choose to call carbon dioxide a pollutant and attempt to regulate it as a toxic substance. Far from being a pollutant, CO2 is plant food and without plants there are no people. But then, politicians and alarmists are nothing if not short sighted, and human hubris knows no bounds.
Be safe, enjoy the interglacial and stay skeptical.